Devices are known for increasing the output of petroleum wells by the gas-lift principle especially during flow-rate-decrease periods, when the pressure of the subterranean reservoir becomes insufficient to entrain the petroleum to the surface. With this aim in view, lift gases under pressure are continously or intermittently introduced, at certain levels, into the tubing or well string.
Another device, meant to entrain the petroleum from the well, uses ejectors mounted inside the tubing, concomitantly a pulverization of the petroleum and its mixing with the lift gas, the dimensions of the annular slot covering the ejection nozzle varying as a function of the conditions within the well.
These devices have the disadvantage of requiring large working pressures and a considerable consumption of lits gases and, when the petroleum is entrained as oil slugs alternating with gas slugs, there is frequently a downslide motion of the petroleum along the inner walls of the tubing, thus reducing the efficiency and increasing the gas consumption.
There is still another device using, to improve the well efficiency and reduce gas consumption, the Coanda effect. This device operates by means of pumping the lift gas through an annular space, previously sealed with a packer over the orifices. The gas is introduced into the tubing at different levels, through some annular slots having adjustable openings. Above these slots there is a Venturi nozzle allowing the fluid jet deflection along the inner walls of the nozzle due to the Coanda effect and the active upward entrainment of the fluid, concomitantly with the dispersal of the slugs by their pulverization into the mass of the lift gas.
The disadvantage of this device is that it has a low efficiency under the circumstances of an unsteady functioning and of a great consumption of lift gases.